Platelet glycoprotein (GP) IIb-IIIa inhibitors may become useful antithrombotic agents. Ro 4–5054 is a low molecular weight, noncyclic, peptidomimetic inhibitor that is three orders of magnitude more potent than RGDS in inhibiting fibrinogen binding to purified GPIIb-IIIa and in preventing platelet aggregation. Comparisons of RGDS and Ro 4–5054 in cell adhesion assays showed that, in contrast to RGDS, Ro 4–5054 was highly selective GPIIb-IIIa inhibitor. Effects of RGDV and Ro 4– 5054 on the conformation and activation state of GPIIb-IIIa were also examined. RGDV and Ro 4–5054 induced conformational changes in purified inactive GPIIb-IIIa as determined by binding of the monoclonal antibody D3GP3 (D3). These conformational alterations were not reversed after inhibitor removal, as indicated by the continued exposure of the D3 epitope and a newly acquired ability to bind fibrinogen. Similarly, RGDV and Ro 4–5054 induced conformational changes in GPIIb-IIIa on the intact platelet. However, after removal of the inhibitors, exposure of the D3 epitope was fully reversed and the platelets did not aggregate in the absence of agonist. Thus, while RGD(X) peptides and Ro 4–5054 transformed purified inactive GPIIb-IIIa into an irreversibly activated conformer, the effects of these inhibitors were reversible on the intact platelet. This suggests that factors present in the platelet membrane or cytoplasm may regulate in part the ability of the complex to shift between active and inactive conformers.